The present invention relates to a switching element of foil construction, which, when triggered, generates a signal dependent on the size of the triggered area.
Such a switching element of foil construction embodies a first carrier-foil, to which a triggering layer of a first resistive material, e.g. graphite, is applied, and a second carrier-foil, to which a sensor layer of a second resistive material, e.g. a semiconductor material, is applied. The first resistive material and the second resistive material are tuned to each other in such a way that, when there is contact between the triggering layer and the sensor layer, the boundary layer between the triggering layer and the sensor layer is essentially governed by the expansion of the area of contact.
The first carrier-foil and the second carrier-foil are arranged a certain distance from each other by means of spacers, in such a way that the triggering layer and the sensor layer are opposite and, when the switching element has not been operated, are not in contact with each other. When the switching element is triggered or operated, the triggering layer and the sensor layer are moved towards each other in opposition to the resetting force of the carrier-foils and come into contact with each other. With small triggering forces, the two layers are in contact with each other at a first point of their surface; the area of contact increases as the pressure on the switching element is increased.
If the electrical resistance of the switching element is measured, a characteristic quantity is obtained which is a directly dependent on the area of mutual contact, and which. taking account of the resetting force of the carrier-foils, permits conclusions to be drawn concerning the triggering forces acting on the switching element. For this reason, such switching elements can be used as pressure sensors, for example.
Such pressure sensors can be manufactured cost-effectively and have proved to be extremely robust and reliable in practice. The triggering performance and the dynamics of such pressure sensors are, however, unsuitable for certain applications. Whereas, in the case of sensors which are generally round, the radial expansion of the triggered area is essentially a linear function of the force exerted on the switching element, an essentially quadratic dependence is obtained for the contact area. The resistance behaviour of the sensor as a function of the triggering force consequently exhibits a characteristic determined by this quadratic dependence, which renders the sensors unsuitable for particular applications.
The problem of the present invention is consequently to propose such a switching element of foil construction which enables the triggering performance to be matched to the application concerned.
According to the invention, this problem is solved by a switching element of foil construction, with a first carrier-foil, to which a triggering layer of a first resistive material is applied, wherein the triggering layer has a first electrical terminal, and a second carrier-foil, to which a sensor layer consisting of a second resistive material is applied, wherein the sensor layer has a second electrical terminal. The first carrier-foil and the second carrier-foil are arranged a certain distance from each other by means of spacers, in such a way that the triggering layer and the sensor layer are opposite each other and, when the switching element is not operated, are not in contact with each other, whereas, when the switching element is triggered, the triggering layer and the sensor layer are initially in contact with each other at a first point of their surface, and the area of contact increases as the pressure on the switching element is increased. The first resistive material and the second resistive material are tuned to each other in such a way that, when there is contact between the triggering layer and the sensor layer, the resistance of the boundary layer between the triggering layer and the sensor layer is essentially determined by the size of the contact area. According to the invention, the sensor layer is designed in such a way that, starting from the first point, its electrical resistivity varies with the distance from the first point in the direction of increasing contact-area, in such a way that a predetermined triggering behaviour of the switching element as a function of the compressive force acting on the switching element is obtained.
Besides being determined by the resistance of the boundary layer between the triggering layer and the sensor layer, the triggering behaviour of such a switching element is also determined by the resistance in the sensor layer between the triggering point and the second electrical terminal. An electrical signal, e.g. an electrical voltage, applied to the sensor layer via the boundary layer at a triggering point, must in fact be dissipated via the resistance section between the triggering point and the second terminal.
By deliberate variation of the resistivity across this resistance section, the voltage drop in the resistance section can consequently be influenced as a function of the triggering point, so that the triggering performance of the switching element can be linearised, for example. Such a switching element can consequently be optimised in respect of its triggering performance, i.e. its dynamics, for any application.
In a preferred development of the switching element, the varying resistivity is produced by the deliberate addition of a third resistive material to the second resistive material, wherein the resistivity of the third resistive material and the resistivity of the second resistive material are different from each other, and wherein the concentration of the third resistive material varies with the distance from the first point. The variation of the resistivity can be brought about, for example, by adding a low-resistance material, e.g. silver, to a high-resistance semiconductor material, wherein the resistivity of the sensor layer becomes smaller as the quantity of added material is increased. Conversely, the variation can also be brought about by adding a high-resistance material to a layer of low-resistance material.
The third resistive material is preferably added to the second resistive material in the form of local inclusions. This kind of addition permits simple manufacture of the sensor layer, at the same time as good control of the concentration of the third resistive material in the sensor layer. The dependence of the concentration of the third resistive material can, for example, be brought about by a particular spatial arrangement of inclusions of equal extent or by a regular spatial arrangement of inclusions with different extents, or by a combination of the two.
The second resistive material preferably exhibits a semiconductor material, and the third resistive material has an appreciably lower resistance than the second resistive material. The semiconductor material can, for example, incorporate semiconductor ink used in the manufacture of foil pressure-sensors, with which the required area-effect can be advantageously brought about at the boundary layer with a graphite triggering layer, while the third resistive material includes silver.
In the manner described above, the resistivity of the sensor layer, starting from the first point, for example the centre of a round switching element, can increase in a radial direction in proportion to the distance from the first point. The distances chosen are derived from the desired sensor dynamics.
The inclusions are advantageously electrically insulated from the second electrical connection terminal. This prevents the switching element from completely switching through as a result of inclusions extending into the boundary layer between triggering layer and sensor layer, thereby rendering pressure detection impossible.
In addition, the inclusions are preferably completely covered by the second resistance material on the side facing the triggering layer. The covering layer consisting of the second resistive material on the one hand prevents direct switching-through of the triggering layer to the inclusions, and on the other acts as a protective layer against mechanical damage.
The triggering layer of the switching element can include a resistive material with a uniform resistivity. This can, for example, be a graphite layer, which can easily be produced in a screen printing process. In an alternative development, the triggering layer can be built up similar to the sensor layer, i.e. the triggering layer exhibits a resistivity which, starting from the first point, varies with the distance from the first point in the direction of increasing contact-area. The characteristic of the resistivity in the triggering layer can correspond to the characteristic of the resistivity in the sensor layer or can exhibit a completely different characteristic.